Liquid crystal display device and manufacturing method thereof

ABSTRACT

A liquid crystal display device has a liquid crystal display panel including pixels each having an active device, a pixel electrode, a common electrode and a liquid crystal layer arranged in a dot matrix array. The liquid crystal display panel has a first substrate, a second substrate, and a liquid crystal layer provided between the first substrate and the second substrate. The first substrate has the active device, the pixel electrode, the common electrode and a first alignment film. The second substrate has a second alignment film. The first alignment film and the second alignment film are respectively a photo alignment film formed by irradiating a photo decomposition type insulating film with light. The second alignment film has a thickness of at least 10 nm and no greater 50 nm and is thinner than the first alignment film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/689,731, filed on Apr. 17, 2015, which is a continuation of U.S.patent application Ser. No. 14/557,581, filed on Dec. 2, 2014 (now U.S.Pat. No. 9,036,120), which is a continuation of U.S. application Ser.No. 14/107,028, filed on Dec. 16, 2013 (now U.S. Pat. No. 8,908,133),which, in turn, is a continuation of U.S. application Ser. No.13/926,247, filed on Jun. 25, 2013 (now U.S. Pat. No. 8,619,221), whichin turn, is a continuation application of U.S. application Ser. No.13/723,268, filed on Dec. 21, 2012 (now U.S. Pat. No. 8,493,532), whichin turn is a continuation of U.S. application Ser. No. 13/325,314, filedon Dec. 14, 2011 (now U.S. Pat. No. 8,339,549), which, in turn, is acontinuation of U.S. application Ser. No. 12/948,899, filed on Nov. 18,2010 (now U.S. Pat. No. 8,085,372), the contents of which areincorporated herein by reference.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent ApplicationJP 2009-263616 filed on Nov. 19, 2009, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION Description of Related Art

The present invention relates to a liquid crystal display device and amanufacturing method thereof, and more particularly, to a techniqueeffectively applicable to an IPS (In-Plane Switching) mode liquidcrystal display device using a photo decomposition type alignment filmto control the alignment of a liquid crystal layer.

Conventionally, an active matrix driving type liquid crystal displaydevice (hereinbelow, simply referred to as a “liquid crystal displaydevice”) is widely used in a liquid crystal television set, a liquidcrystal display for a personal computer, a liquid crystal display for amobile electronic device and the like.

These liquid crystal display devices respectively have a liquid crystaldisplay panel where a liquid crystal layer is encapsulated between apair of substrates. The liquid crystal display panel has a displayregion having plural pixels in a dot matrix array. In each pixel, whichhas an active device, a pixel electrode, a common electrode and a liquidcrystal layer, the alignment of the liquid crystal layer is changed andthe light transmittance is changed in accordance with the level ofpotential difference between the pixel electrode and the commonelectrode. Further, the methods of arrangement of the pixel electrodeand the common electrode are briefly classified into providing theseelectrodes on different substrates and providing the electrodes on thesame substrate.

As a liquid crystal layer operation mode (alignment changing method) ina liquid crystal display panel where the pixel electrode and the commonelectrode are provided on different substrates, e.g., TN (TwistedNematic) mode, STN (Super Twisted Nematic) mode and VA (VerticallyAligned or Vertical Alignment) mode are well known. Further, as a liquidcrystal layer operation mode in a liquid crystal display panel where thepixel electrode and the common electrode are provided on the samesubstrate, for example, the IPS mode and FFS (Fringe Field Switching)mode are well known.

In the IPS mode liquid crystal display panel, the alignment of theliquid crystal layer is homogeneous when there is no potentialdifference between the pixel electrode and the common electrode. Uponapplication of potential difference between the pixel electrode and thecommon electrode, a so-called lateral electric field mainly having acomponent parallel to the substrate plane is applied to the liquidcrystal layer, thereby the alignment of the liquid crystal layer ischanged. At this time, the change of the alignment of the liquid crystallayer is made mainly by rotation of liquid crystal molecules in a planeapproximately parallel to the substrate plane and the change of the tiltangle of the liquid crystal molecules is small. Accordingly, in the IPSmode liquid crystal display panel, as the change of effective value ofretardation accompanying voltage application is small, a display withexcellent tone reproduction in a wide view angle range can be produced.

In addition to the IPS mode liquid crystal display panel, conventionalgeneral liquid crystal display panels have an alignment film to controlliquid crystal layer alignment when no electric field is applied.

Conventionally, the alignment film is generally formed by, afterformation of a dielectric film such as a polyimide film, performingrubbing processing on the surface of the dielectric film.

However, the formation of the alignment film by performing the rubbingprocessing on the surface of the dielectric film has a problem that someof the dielectric film peeled by the rubbing processing remains and ismixed with the liquid crystal layer which causes deterioration ofdisplay quality.

Accordingly, in the recent manufacturing method of liquid crystaldisplay panel, the alignment film is formed by irradiation of the photodecomposition type dielectric film with predetermined light (forexample, ultraviolet light having an emission line in a 240 nm to 400 nmwavelength band).

However, when the alignment film formed by irradiation of the dielectricfilm with light (hereinbelow, referred to as a “photo alignment film”)is used, it is necessary to increase the amount of light irradiation toobtain practical alignment (for example, homogeneousness of thealignment of the liquid crystal layer or the like when no electric-fieldis applied). Accordingly, the conventional photo alignment film isgenerally colored in yellow, and the light transmittance is lowered.Therefore, in the liquid crystal display panel having the photoalignment film, the light transmittance in each pixel is lowered by thereduction of the light transmittance in the photo alignment film.

In the IPS mode liquid crystal display panel, to prevent the reductionof the light transmittance due to coloration of the photo alignmentfilm, e.g., a method of reducing the amount of light irradiation information of a photo alignment film on the substrate without the activedevice, the pixel electrode and the like (hereinbelow, referred to as an“opposite substrate”) of the pair of substrates to a smaller amount thanthe amount of irradiation in formation of the other photo alignment filmon the substrate having the active device, the pixel electrode and thelike (hereinbelow, referred to as a “TFT substrate”), has been proposed(for example, see JPA NO. 2007-033672).

However, in the IPS mode liquid crystal display panel, when the amountof light irradiation is small in formation of the photo alignment filmon the opposite substrate side, the alignment of the photo alignmentfilm is deteriorated, and an after image easily occurs due to thedeterioration of the alignment.

Accordingly, when the amount of light irradiation is small in formationof the photo alignment film on the opposite substrate side, it isnecessary to control the amount of light irradiation such that an afterimage occurs within an allowable range.

In the liquid crystal display panel disclosed in JPA NO. 2007-033672, asan example of the amount of light irradiation in formation of the photoalignment film on the opposite substrate side, with 30% of the amount ofirradiation in formation of the photo alignment film on the TFTsubstrate side as a lower limit, the amount of light irradiation isdesirably 40% to 50%.

That is, the conventional IPS mode liquid crystal display panel having aphoto alignment film has a problem that it is difficult to achieve bothsuppression of reduction of light transmittance due to coloration of thephoto alignment film and suppression of occurrence of after image due tothe reduction of alignment of the photo alignment film.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation, and provides a technique to achieve both improvement in lighttransmittance and suppression of occurrence of after image in an IPSmode liquid crystal display panel having e.g. a photo alignment film.

The above-described features and novel features of the present inventionwill be made clear from the descriptions of the specification and theaccompanying drawings.

Among aspects of the present invention disclosed in the presentapplication, the outline of typical aspect is as follows.

According to an aspect of the present invention, there is provided aliquid crystal display device having a liquid crystal display panelincluding pixels each having an active device, a pixel electrode, acommon electrode and a liquid crystal layer arranged in a dot matrixarray, the liquid crystal display panel having a first substrate, asecond substrate, and a liquid crystal layer provided between the firstsubstrate and the second substrate, the first substrate having theactive device, the pixel electrode, the common electrode and a firstalignment film, the second substrate having a second alignment film, andthe first alignment film and the second alignment film beingrespectively a photo alignment film formed by irradiating a photodecomposition type insulating film with light, wherein the secondalignment film is thinner than the first alignment film, and has athickness of equal to or greater than 10 nm and equal to or less than 50nm.

In the above-described liquid crystal display apparatus, the firstalignment film has a thickness of equal to or greater than 80 nm andequal to or less than 130 nm.

According to another aspect of the present invention, there is provideda liquid crystal display device manufacturing method including: a firststep of forming a first substrate having a first alignment film; asecond step of forming a second substrate having a second alignmentfilm; and a third step of attaching the first substrate and the secondsubstrate to each other, and encapsulating a liquid crystal layerbetween the pair of substrates, the first alignment film and the secondalignment film being respectively formed by performing alignmentprocessing to irradiate a photo decomposition type insulating film withlight on a previously determined condition, wherein the first step has:a step of forming a first thin film laminated body having a plurality ofscanning signal lines, a plurality of video image signal lines, aplurality of active devices, a plurality of pixel electrodes, a commonelectrode, and a plurality of insulating layers, on a first insulatingsubstrate; and a step of forming the first alignment film on the firstthin film laminated body, and wherein the second alignment film isformed to be thinner than the first alignment film, and to have athickness after the alignment processing equal to or greater than 10 nmand equal to or less than 50 nm.

In the above-described the liquid crystal display apparatusmanufacturing method, the first alignment film is formed to have athickness after the alignment processing equal to or greater than 80 nmand equal to or less than 130 nm.

Further, in the above-described the liquid crystal display apparatusmanufacturing method, an amount of light irradiated to perform thealignment processing on the second alignment film is equal to or greaterthan 10% and equal to or less than 50% of an amount of light irradiatedto perform the alignment processing on the first alignment film.

Further, in the above-described the liquid crystal display apparatusmanufacturing method, the second step has: a step of forming a secondthin film laminated body, having a plurality of color filters and aplanarized layer, on the second insulating substrate; and a step offorming the second alignment film on the second thin film laminatedbody.

In accordance with the liquid crystal display device and themanufacturing method of the liquid crystal display device according tothe present invention, it is possible to achieve both improvement inlight transmittance and suppression of occurrence of after image in aliquid crystal display panel having a photo alignment film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plane view showing an example of a plane structureof pixels in a liquid crystal display panel according to a firstembodiment of the present invention;

FIG. 2 is a schematic cross-sectional view showing an example of across-sectional structure along a line A-A′ in FIG. 1;

FIG. 3 is a schematic cross-sectional view showing an example of across-sectional structure along a line B-B′ in FIG. 1;

FIG. 4 is a schematic cross-sectional view showing an example of anoperation of pixels (liquid crystal layer) in the liquid crystal displaypanel according to the first embodiment;

FIG. 5 is a graph showing an example of relation between thickness of analignment film and light transmittance;

FIG. 6 is a graph showing an example of relation between the thicknessof the alignment film and AC after image intensity; and

FIG. 7 is a graph for explanation of the essential point of the liquidcrystal display panel according to a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, the present invention will be described in detail withembodiments in accordance with the accompanying drawings.

Note that in all the figures for explanation of the embodiments,elements having the same function will have the same reference numeralsand repeated explanations thereof will be omitted.

First Embodiment

FIGS. 1 to 4 schematically illustrate the structure of a liquid crystaldisplay panel according to a first embodiment of the present invention.

FIG. 1 is a schematic plane view showing an example of a plane structureof pixels in a liquid crystal display panel according to the firstembodiment of the present invention. FIG. 2 is a schematiccross-sectional diagram showing an example of a cross-sectionalstructure along a line A-A′ in FIG. 1. FIG. 3 is a schematiccross-sectional diagram showing an example of the cross-sectionalstructure along a line B-B′ in FIG. 1. FIG. 4 is a schematiccross-sectional diagram showing an example of an operation of pixels(liquid crystal layer) in the liquid crystal display panel according tothe first embodiment.

In the first embodiment, as an example of the liquid crystal displaydevice according to the present invention, a liquid crystal displaydevice of active matrix driving type having the IPS mode as a liquidcrystal layer operation mode is used. Note that the present inventionrelates to the structure of the liquid crystal display panel of theliquid crystal display device and more particularly relates to thestructure of an alignment film. The other elements than the structure ofthe alignment film may be basically any of well-known elements.Accordingly, in the following description of the present specification,only the structure of the liquid crystal display panel of the liquidcrystal display device according to the present invention will bedescribed.

The liquid crystal display panel according to the present invention hase.g. a first substrate 1, a second substrate 2, a liquid crystal layer3, a first polarizing plate 4 and a second polarizing plate 5 as shownin FIGS. 1 to 3. Further, the liquid crystal display panel shown inFIGS. 1 to 3 is a so-called transmission type liquid crystal displaypanel to display a video image, a still image or the like by modulationof light from a backlight. Light 6 from the backlight enters e.g. fromthe first polarizing plate 4 side into the liquid crystal display panel.At this time, the amount of the light 6 passing through the liquidcrystal display panel is changed in accordance with relation between thepolarized status of the light 6 passed through the first polarizingplate 4 and the liquid crystal layer 3 and the direction of thetransmission axis (absorption axis) of the second polarizing plate 5.

The first substrate 1 has a first insulating substrate 7, a first thinfilm laminated body formed on the first insulating substrate 7, and afirst alignment film 8 formed on the first thin film laminated body. Thefirst insulating substrate 7 is a transparent insulating substrate suchas a glass substrate. The first thin film laminated body has pluralscanning signal lines 9, a first insulating layer 10, plural video imagesignal lines 11, plural active devices 12, a second insulating layer 13,a common electrode 14, a third insulating layer 15, and a pixelelectrode 16. Further, as described later, the first alignment film 8 isa photo alignment film formed by irradiating a photo decomposition typeinsulating film with ultraviolet light.

The second substrate 2 has a second insulating substrate 17, a secondthin film laminated body formed on the second insulating substrate 17,and a second alignment film 18 formed on the second thin film laminatedbody. The second insulating substrate 17 is a transparent insulatingsubstrate such as a glass substrate. The second thin film laminated bodyhas e.g. a black matrix (light shielding film) 19, a color filter 20,and a planarized layer 21. Further, as described later, the secondalignment film 18 is a photo alignment film formed by irradiating aphoto decomposition type insulating film with ultraviolet light.

The liquid crystal display panel in the first embodiment has a displayregion having plural pixels in a dot matrix array, and each pixel hasthe liquid crystal layer 3, the active device 12, the pixel electrode16, and the common electrode 14 provided on the first substrate 1. Notethat FIG. 1 shows the plane structure of three pixels arrayed along anextending direction (x-direction) of the scanning signal line 9.

Further, when the liquid crystal display panel is available for RGBcolor display, the color filter 20 of each pixel is any one of a redcolor filter to transmit only red color light, a green color filter totransmit only green color light and a blue color filter to transmit onlyblue color light. Further, at this time, 1 dot (picture element) of avideo image or a still image includes three pixels, i.e. a pixel havingthe red color filter, a pixel having the green color filter and a pixelhaving the blue color filter. The three pixels are arrayed in e.g. theextending direction (x-direction) of the scanning signal line 9.

The active device 12 is a TFT device with a part of the scanning signalline 9 as a gate electrode. A semiconductor layer (not shown) laminatedvia the first insulating layer 10 is provided on the gate electrode.Further, a first source-drain electrode 12 s and a part of the videoimage signal line (second source-drain electrode) are connected to thesemiconductor layer. At this time, the first source-drain electrode 12 sis connected to the pixel electrode 16 with a contact hole CH.

Further, at this time, the pixel electrode 16 and the common electrode14 are laminated via the third insulating layer 15. The pixel electrode16 which is closer to the liquid crystal layer 3 has a comb teeth planeshape. Note that in the example shown in FIG. 1, the direction in whichthe teeth of the pixel electrode 16 extend is parallel to a direction inwhich the video image signal line 11 extends (y-direction). However, theteeth extending direction is not limited to the above direction but maybe another direction.

The first alignment film 8 and the second alignment film 18 areinsulating films to control the alignment of the liquid crystal layer 3when no electric field is applied, i.e., when there is no potentialdifference between the pixel electrode 16 and the common electrode 14.Further, in the case of an IPS mode liquid crystal display panel, thealignment of the liquid crystal layer 3 when no electric field isapplied is homogeneous. At this time, assuming that the comb teeth planeshape of the pixel electrode 16 is as shown in FIG. 1, the firstalignment film 8 and the second alignment film 18 are formed such thatan acute angle α formed with the longitudinal direction of the liquidcrystal molecule 3M when no electric field is applied and the extendingdirection of the scanning signal line 9 (x-direction) is 75° to 85°.

In the pixel having the above structure, when a potential difference isapplied between the pixel electrode 16 and the common electrode 14, anarch shaped electric line of force 22 passing through the liquid crystallayer 3 and connecting these electrodes is formed as shown in e.g. FIGS.3 and 4. At this time, an electric field (so-called lateral electricfield) mainly having a component parallel to the substrate plane (xyplane) is applied to the liquid crystal layer 3. When the lateralelectric field is applied to the liquid crystal layer 3 in homogeneousalignment, an alignment change characteristic of IPS mode occurs. Thatis, as the alignment change, the alignment direction of the liquidcrystal layer 3 (longitudinal direction of the liquid crystal molecule3) is mainly rotated in the substrate plane.

Further, the alignment change of the liquid crystal layer 3 occurs in apart to which the lateral electric field is applied, i.e., a region BLshown in FIG. 4, and the change is propagated in a thickness direction(z-direction) of the liquid crystal layer 3. At this time, in the liquidcrystal layer 3, rotation of the liquid crystal molecule 3M, around aboundary surface with respect to the first alignment film 8 and around aboundary surface with respect to the second alignment film 18, in thesubstrate plane, is suppressed by the influence of an alignmentrestraining force with these alignment films. As a result, the alignmentstatus of the liquid crystal layer 3 when the lateral electric field isapplied is twisted alignment as shown in FIG. 4.

The first alignment film 8 and the second alignment film 18 in theliquid crystal display panel in the first embodiment are photo alignmentfilms respectively formed by irradiating a photo decomposition typeinsulating film with ultraviolet light as described above. A moreparticular method of forming the photo alignment film is e.g. asfollows.

First, to obtain a photo decomposition type insulating material used information of the photo alignment film, e.g. 1.0 mol % p-phenylenediamine is solved in N-methyl-2-pyrrolidone, then 1 mol % 2-cyclobutanetetra carboxylic anhydrate is added to the solution to cause reactionfor 12 hours at 20° C., thereby poly amic acid varnish, in whichstandard polystyrene equivalent weight average molecular weight is about100,000 and weight average molecular weight/number average molecularweight (Mv/Mn) is about 1.6, is obtained.

Next, the poly amic acid varnish is diluted to 6% concentration, thensolid content 0.3% by weight of γ-amino propyl triethoxy silane isadded, and then printed on the first thin film laminated body and thesecond thin film laminated body, and heated at 210° C. for 30 minutes,thereby a photo decomposition type insulating film (polyimide film) isformed.

Thereafter, alignment processing of irradiating the photo decompositiontype polyimide film with light (ultraviolet light) from a polarizing UVlamp having an emission line in a wavelength band from e.g. 240 nm to400 nm is performed. The alignment processing is performed byirradiating the ultraviolet light from e.g. a high pressure mercurylamp, as linear polarized light at a polarization rate of about 20:1,using a pile polarizer formed with laminated quartz substrates, atirradiation energy of about 4 J/cm². At this time, the direction of thelinear polarized light is orthogonal to the longitudinal direction ofthe liquid crystal molecule 3M when no electric field is applied.

In the case where the first alignment film 8 and the second alignmentfilm 18 are photo alignment films formed in accordance with theabove-described procedure, when the conventional forming method is used,these alignment films are colored in yellow as described above.

Further, the inventors of the present invention have found that in thecase where the first alignment film 8 and the second alignment film 18are photo alignment films formed in accordance with the above-describedprocedure, the coloration by irradiation with light (ultraviolet light)occurs not only in the alignment film but also in an insulating filmformed of organic material such as the planarized layer 21. Further, theinventors of the present invention have manufactured a liquid crystaldisplay panel using the first substrate 1 where the first thin filmlaminated body is formed of only inorganic material and the secondsubstrate 2 where only the planarized layer 21 of the second thin filmlaminated body is formed of organic material, and measured the degree ofcoloring. It has been found as a result that the contribution ratio toreduction of transmittance in the first alignment film 8, that in thesecond alignment film 18, and that in the planarized layer 21 areapproximately equal to each other (i.e., 1:1:1).

That is, the coloration of the first alignment film 8 and the secondalignment film 18 greatly contributes to reduction of transmittance ofthe liquid crystal panel. Accordingly, when the first alignment film 8and the second alignment film 18 are photo alignment films, it isnecessary to suppress the amount of coloration of these alignment filmsto prevent reduction of the transmittance of the liquid crystal displaypanel.

In the conventional photo alignment film forming method, to suppress theamount of coloration, as described above, the amount of lightirradiation in formation of e.g. the photo alignment film (secondalignment film 18) of the second substrate 2 is smaller than the amountof light irradiation in formation of the photo alignment film (firstalignment film 8) of the first substrate 1. In this method, however, asdescribed above, it is difficult to achieve both of suppression ofreduction of light transmittance due to coloration of alignment film andsuppression of occurrence of after image due to deterioration ofalignment.

On the other hand, in the liquid crystal display panel in the firstembodiment, as shown in e.g. FIG. 4, the thickness D2 of the photoalignment film (second alignment film 18) of the second substrate 2 isthinner than the thickness D1 of the photo alignment film (firstalignment film 8) of the first substrate 1, thereby it is possible toachieve both suppression of the reduction of light transmittance due tocoloration of the alignment film and suppression of occurrence of afterimage due to deterioration of alignment.

FIGS. 5 and 6 are schematic diagrams for explanation of preferablevalues of the thickness of the first alignment film and the secondalignment film in the liquid crystal display panel in the firstembodiment.

FIG. 5 schematically shows an example of relation between the thicknessof the alignment film and the light transmittance. FIG. 6 schematicallyshows an example of relation between the thickness of the alignment filmand AC after image intensity.

Note that FIG. 5 is a graph with a lateral axis as the thickness D (nm)of the alignment film, and a vertical axis as light transmittanceTR_(ORI) (%). Further, in FIG. 5, rhombic points indicate the results ofmeasurement by the inventors of the present invention, and aright-downward straight line, a regression straight line obtained fromthe results of measurement.

Further, FIG. 6 is a graph with a lateral axis as the thickness D (nm)of the alignment film, and a vertical axis as AC after image intensityIoS_(AC)(%). Further, in FIG. 6, outline rhombic points indicate theresults of measurement of the AC after image intensity IoS_(AC) when thethickness D1 of the first alignment film 8 is changed while thethickness D2 of the second alignment film 18 is fixedly 100 nm, and adotted curve indicates a regression curve obtained from the results themeasurement. Further, in FIG. 6, outline round points indicate theresults of measurement of the AC after image intensity IoS_(AC) when thethickness D2 of the second alignment film 18 is changed while thethickness D1 of the first alignment film 8 is fixedly 100 nm, and asolid curve indicates a regression curve obtained from the results ofmeasurement.

The inventors of the present invention have examined the relationbetween the thickness of the photo alignment film and the lighttransmittance, and obtained e.g. the results as shown in FIG. 5. Notethat FIG. 5 shows the relation between the thickness of a photoalignment film and the light transmittance when only the photo alignmentfilm is formed on a glass substrate having the same thickness as that ofthe first insulating substrate 7 or the second insulating substrate 18.Further, the photo alignment film is formed in accordance with theabove-described procedure. The irradiation amount of light (ultravioletlight) is a constant amount (e.g., light at irradiation energy of 4mW/cm² is irradiated for 16 minutes 40 seconds to achieve accumulatedirradiation amount of 4 J/cm²) regardless of thickness.

As it is apparent from FIG. 5, even when the light irradiation amount isconstant, the light transmittance TR_(ORI) of the photo alignment filmincreases as the thickness D of the photo alignment film becomesthinner. Accordingly, it is considered that the reduction of lighttransmittance due to coloration can be suppressed without impairing thealignment of these alignment films by thinning the first alignment film8 and the second alignment film 18.

Accordingly, when the degree (intensity) of an after image when thefirst alignment film 8 and the second alignment film 18 are thinned isabout the same as that when the conventional first alignment film andthe second alignment film have a general thickness (e.g. about 100 nm),it is considered that the reduction of light transmittance due tocoloration of alignment film can be suppressed and the occurrence ofafter image due to deterioration of alignment can be suppressed.

Then, the inventors of the present invention have examined the degree(intensity) of an after image when the first alignment film 8 and thesecond alignment film 18 are thinned, and obtained e.g. the results asshown in FIG. 6. Note that in FIG. 6, in a case where a window patternat a maximum brightness is displayed on a screen for 30 minutes and thepattern on the screen with the brightness changed to 10% of the maximumbrightness is displayed for 2 minutes, the luminance fluctuationΔB/B_(10%) between an after image part of the window pattern andbrightness B in a peripheral intermediate halftone part is shown as theAC after image intensity IoS_(AC).

The AC after image intensity IoS_(AC) on the vertical axis in the graphshown in FIG. 6 is the intensity of an after image as an index of afterimage in the IPS mode liquid crystal display panel, which is related toe.g. the alignment restraining force of the liquid crystal layer 3 withthe first alignment film 8, and the alignment restraining force of theliquid crystal layer 3 with the second alignment film 18.

As it is apparent from FIG. 6, when the thickness of the first alignmentfilm 8 is changed while the thickness of the second alignment film 18 isfixed, within a range where the thickness D1 of the first alignment film8 is equal to or less than 100 nm, the AC after age intensity IoS_(AC)is increased as the thickness D1 becomes thinner. In this manner, it isconsidered that in the range where the thickness D1 of the firstalignment film 8 is equal to or less than 100 nm, the AC after imageintensity IoS_(AC) depends on the thickness because when the thicknessD1 of the first alignment film 8 is thinner, an electric field appliedaround the surface boundary of the liquid crystal layer 3 with respectto the first alignment film 8 is relatively intensified and the twist ofthe liquid crystal molecule 3M is increased.

On the other hand, as it is apparent from FIG. 6, when the thickness ofthe second alignment film 18 is changed while the thickness of the firstalignment film 8 is fixed, even when the thickness D2 of the secondalignment film 18 is within the range of 100 nm, the AC after imageintensity IoS_(AC) is approximately constant equal to or less than 1.0%.In this manner, it is considered that the AC after image intensityIoS_(AC) is approximately constant regardless of the thickness D2 of thesecond alignment film 18 because no electric field is applied to aroundthe boundary surface of the liquid crystal layer 3 with respect to thesecond alignment film 18 and the change of the thickness D2 of thesecond alignment film 18 does not influence the degree of twist of theliquid crystal molecule 3M.

To obtain a practical after image characteristic in the IPS mode liquidcrystal display device, it is necessary to suppress the AC after imageintensity IoS_(AC) to or less than 1.0%. Therefore, to suppress thereduction of transmittance due to coloration and suppress the occurrenceof after image, as it is understood from FIG. 6, it is desirable thatthe thickness D1 of the first alignment film 8 is as thin as possiblebut equal to or greater than 80 nm, e.g., equal to or greater than 80 nmand equal to or less than 130 nm. Note that the first alignment film 8is formed by printing or coating photo decomposition insulating materialas described above. Accordingly, in consideration of thickness variationoccurred in formation of the first alignment film 8, the thickness D1 ofthe first alignment film 8 is desirably equal to or greater than 90 nmand equal to or less than 110 nm.

Further, at this time, it is desirable that the thickness D2 of thesecond alignment film 18 is as thin as possible. However, as describedabove, the second alignment film 18 is formed by printing or coatingphoto decomposition type insulating material. Accordingly, when thethickness D2 of the second alignment film 18 is too thin, an openingdefect such as a so-called pin hole often occurs. According to theresearch by the inventors of the present invention, the number (density)of pin holes when the thickness D2 of the second alignment film 18 is 10nm is within an allowable range, while when the number (density) of pinholes when the thickness D2 is 5 nm is without the allowable range, andthe alignment of the liquid crystal layer 3 when no electric field isapplied is deteriorated. Accordingly, it is desirable that the thicknessD2 of the second alignment film 18 is e.g. equal to or greater than 10nm to equal to or less than 50 nm.

Based on the above-described study, the inventors of the presentinvention have manufactured plural liquid crystal display panels withdifferent combinations of the thickness D1 of the first alignment film 8and the thickness D2 of the second alignment film 18, then compared thelight transmittance and the AC after image intensities, and obtained theresults as shown in the following Table 1.

TABLE 1 PT D1 (nm) D2 (nm) TR_(LCD) (%) ΔTR (%) IoS_(AC) (%) PT1 100 1004.49 — 0.7 PT2 50 50 4.66 +0.17 1.3 PT3 100 50 4.57 +0.08 0.7 PT4 100 104.64 +0.15 0.7 PT5 80 10 4.67 +0.18 0.7

Note that five types of liquid crystal display panels PT1 to PT5 shownin the Table 1 are manufactured with the same condition regardingconstituents except that the combinations of the thickness D1 of thefirst alignment film 8 and the thickness D2 of the second alignment film18 are different. Further, the liquid crystal layer 3 is formed byvacuo-injecting a nematic liquid crystal composition A, with a positiveanisotropic dielectric constant Δ∈ having a value of 10.2 (1 kHz, 20°C.), an anisotropic refraction factor Δn having a value of 0.075(wavelength of 590 nm, 20° C.), a twist elastic constant K2 having avalue of 7.0 pN, and a nematic-isotropic phase transition temperature T(N−1) of about 76° C., and sealing with a sealing material ofultraviolet curing resin. Further, at this time, the thickness of theliquid crystal layer 3 (cell gap) is 4.8 μm, and retardation Δnd is 0.36μm. Further, the liquid crystal layer 3 is aligned such that thelongitudinal direction of the liquid crystal molecular 3M when noelectric field is applied is inclined by 75 degrees with respect to thedirection of electric field application (the direction in which thescanning signal line 9 extends). Further, the respective absorption axesof the first polarizing plate 4 and the second polarizing plate 5 areorthogonal to each other and the absorption axis of the first polarizingplate 4 is orthogonal to the longitudinal direction of the liquidcrystal molecule 3M when no electric field is applied.

In the Table 1, the liquid crystal display panel PT1 is a liquid crystaldisplay panel as a first comparative example for comparison with theliquid crystal display panel in the first embodiment, where thethickness D1 of the first alignment film 8 and the thickness D2 of thesecond alignment film 18 are 100 nm. Further, the liquid crystal displaypanel as the first comparative example (PT1) is manufactured by theconventional manufacturing method. In the liquid crystal display panelas the first comparative example (PT1), the light transmittance TR_(LCD)is 4.49% and the AC after image intensity IoS_(AC) is 0.7%.

Further, as it is apparent from the Table 1, in the liquid crystaldisplay panel as the second comparative example (PT2) where thethickness D1 of the first alignment film 8 and the thickness D2 of thesecond alignment film 18 are 50 nm, although the light transmittanceTR_(LCD) is improved in comparison with the first comparative example,as the AC after image intensity IoS_(AC) is 1.3%, it is impossible toachieve both the prevention of the reduction of light transmittance andthe suppression of the occurrence of after image.

On the other hand, in the respective liquid crystal display panel (PT3)where the thickness D1 of the first alignment film 8 is 100 nm and thethickness D2 of the second alignment film 18 is 50 nm, the liquidcrystal display panel (PT4) where the thickness D1 of the firstalignment film 8 is 100 nm and the thickness D2 of the second alignmentfilm 18 is 10 nm, the liquid crystal display panel (PT5) where thethickness D1 of the first alignment film 8 is 80 nm and the thickness D2of the second alignment film 18 is 10 nm, the light transmittanceTR_(LCD) is improved in comparison with the first comparative example,and the AC after image intensity IoS_(AC) is 0.7%. The combinations ofthe thickness D1 of the first alignment film 8 and the thickness D2 ofthe second alignment film 18 in these liquid crystal display panels (PT3to PT5) satisfy the above-described combinations (80 nm≦D1≦130 nm, 10nm≦D2≦50 nm). Further, the differences ΔTR between the lighttransmittance TR_(LCD) in the liquid crystal display panel (PT1) as thefirst comparative example and the light transmittance TR_(LCD) in theseliquid crystal display panels (PT3 to PT5) are +0.08%, +0.15% and+0.18%, and as relative values, with the light transmittance TR_(LCD) inthe liquid crystal display panel (PT1) as the first comparative exampleas “1”, 1.02, 1.03 and 1.04. That is, it is considered that in theliquid crystal display panel in the first embodiment, in comparison withthe liquid crystal display panel as the first comparative example, thelight transmittance can be improved by 2% or greater.

Accordingly, in the liquid crystal display panel in the firstembodiment, it is possible to achieve prevention of the reduction oflight transmittance due to coloration of the first alignment film 8 andthe second alignment film 18 and suppression of the occurrence of afterimage.

Second Embodiment

FIG. 7 schematically shows the essential point of the liquid crystaldisplay panel according to a second embodiment of the present invention.

Note that FIG. 7 is a graph with a lateral axis as a relative value RoIR(%) of light irradiation amount in formation of the second alignmentfilm 18, and a vertical axis as the AC after image intensityIoS_(AC)(%). Further, regarding the relative value RoIR (%) of lightirradiation amount, an irradiation amount the same as that in formationof the first alignment film 8 is 100%.

Further, in FIG. 7, outline rhombic points indicate results ofmeasurement of the AC after image intensity IoS_(AC) when the thicknessD2 of the second alignment film 18 is 100 nm and the thickness D1 of thefirst alignment film 8 is 100 nm and when the light irradiation amountin formation of the second alignment film 18 is changed, and a solidcurve indicates a regression curve obtained from the results ofmeasurement. Further, in FIG. 7, outline round points indicate resultsof measurement of the AC after image intensity IoS_(AC) when thethickness D2 of the second alignment film 18 is 50 nm and the thicknessD1 of the first alignment film 8 is 100 and when the light irradiationamount in formation of the second alignment film 18 is changed, and adotted curve indicates a regression curve obtained from the results ofmeasurement. Further, in FIG. 7, outline rectangular points indicateresults of measurement of the AC after image intensity IoS_(AC) when thethickness D2 of the second alignment film 18 is 20 nm and the thicknessD1 of the first alignment film 8 is 100 nm and when the lightirradiation amount in formation of the second alignment film 18 ischanged, and a broken curve indicates a regression curve obtained fromthe results of measurement.

The liquid crystal display panel in the second embodiment is anapplication of the first embodiment, in which when the light irradiationamount in formation of the second alignment film 18 is reduced to anamount smaller than that in formation of the first alignment film 8 asin JPA NO. 2007-033672, thereby the amount of coloration of the secondalignment film 18 is further reduced.

As in the case of the liquid crystal display panel in the firstembodiment, in a case where the thickness D2 of the second alignmentfilm 18 is equal to or greater than 10 nm and equal to or less than 50nm, when the relation between the relative value RoIR of lightirradiation amount in formation of the second alignment film 18 and theAC after image intensity IoS_(AC) is examined, the results as shown ine.g. FIG. 7 are obtained. Note that FIG. 7 shows the relation in theconventional technique (when the thickness of the second alignment film18 is 100 nm) in addition to the relation between the relative valueRoIR of light irradiation amount and the AC after image intensityIoS_(AC) when the thickness D2 of the second alignment film 18 is 50 nm,and 20 nm, satisfying the conditions of the first embodiment.

In the conventional technique, as it is apparent from FIG. 7, when therelative value RoIR of light irradiation amount in formation of thesecond alignment film 18 is less than about 40%, the AC after imageintensity IoS_(AC) is greater than 1.0%.

On the other hand, in a case where the thickness D2 of the secondalignment film 18 is 50 nm, the AC after image intensity IoSAC isgreater than 1.0% when the relative value RoIR of light irradiationamount in formation of the second alignment film 18 is less than about20%. Further, in a case where the thickness D2 of the second alignmentfilm 18 is 20 nm, even when the relative value RoIR of light irradiationamount in formation of the second orientation film 18 is about 10%, theAC after image intensity IoS_(AC) is less than 1.0%.

As a factor of suppression of after image by thinning the thickness D2of the second alignment film 18, although the details are unknown, it isconsidered as follows. When the film strength of the second alignmentfilm 18 is lowered, a shift easily occurs in the direction of alignmentdue to torque of the liquid crystal molecule 3M to which a lateralelectric field is applied, then the alignment is deteriorated and anafter image easily occurs. Since a broken part occurs due to lightirradiation in the photo decomposition type alignment film, the filmstrength after the alignment processing is lower than that before thealignment processing. Generally, the alignment film contains an additivesuch as a coupling agent for improvement in airtight contact with itssubstrate. It is considered that in the case of a thin film, as thedistance to the substrate is short, the reduction of film strength canbe suppressed by the influence of an adhesive force strengthened withthe coupling agent or the like.

That is, when the thickness D1 of the first alignment film 8 and thethickness D2 of the second alignment film 18 satisfy conditions as givenin the first embodiment, a lower limit value of the amount of lightirradiation in formation of the second alignment film 18 can be evenlower than that in the conventional technique.

Based on the above-described study, the inventors of the presentinvention have manufactured plural liquid crystal display panels havingdifferent combinations of thickness D1 of the first alignment film 8 andthe thickness D2 of the second alignment film 18 and different relativevalues RoIR of amount of light irradiation in formation of the secondalignment film 18, compared the light transmittance and AC after imageintensities, and obtained the results as shown in the following Table 2.

TABLE 2 PT D1(nm) D2(nm) RoIR(%) TR_(LCD)(%) ΔTR(%) IoS_(AC)(%) PT1 100100 100 4.49 — 0.7 PT3 100 50 100 4.57 +0.08 0.7 PT6 100 50 50 4.70+0.21 0.9 PT7 100 50 25 4.76 +0.27 1.0 PT8 100 20 10 4.81 +0.32 0.9 PT580 10 100 4.67 +0.18 0.7 PT9 80 20 10 4.84 +0.35 1.0

The liquid crystal panel PT1 in the Table 2 is the liquid crystaldisplay panel as the first comparative example given in the firstembodiment, in which the thickness D1 of the first alignment film 8 is100 nm and the thickness D2 of the second alignment film 18 is 100 nm,and the relative value RoIR of light irradiation amount in formation ofthe second alignment film 18 is 100%. In the liquid crystal displaypanel (PT1) as the first comparative example, the light transmittanceTR_(LCD) is 4.49% and the AC after image intensity IoS_(AC) is 0.7%.

Further, as it is apparent from the Table 2, in the case of the liquidcrystal display panel (PT3) where the thickness D1 of the firstalignment film 8 is 100 nm, the thickness D2 of the second alignmentfilm 18 is 50 nm, the relative value RoIR of light irradiation amount information of the second alignment film 18 is 100%, the lighttransmittance TR_(LCD) is improved in comparison with the firstcomparative example, and the AC after image intensity IoS_(AC) is 0.7%.This liquid crystal display panel (PT3) is a liquid crystal displaypanel satisfying the conditions in the first embodiment in which it ispossible to achieve both prevention of reduction of light transmittanceand suppression of occurrence of after image.

Further, in the case of the liquid crystal display panel (PT6) where thethickness D1 of the first alignment film 8 is 100 nm, the thickness D2of the second alignment film 18 is 50 nm, the relative value RoIR oflight irradiation amount in formation of the second alignment film 18 is50%, in the case of the liquid crystal display panel (PT7) where therelative value RoIR of light irradiation amount in formation of thesecond alignment film 18 is 25%, and in the case of the liquid crystaldisplay panel (PT8) where the relative value RoIR of light irradiationamount in formation of the second alignment film 18 is 10%,respectively, the light transmittance TR_(LCD) is improved in comparisonwith the liquid crystal display panel as the first embodiment (PT3), andthe AC after image intensity IoS_(AC) is equal to or less than 1.0%.

Similarly, in the case of the liquid crystal display panel (PT9) wherethe thickness D1 of the first alignment film 8 is 80 nm, the thicknessD2 of the second alignment film 18 is 20 nm, the relative value RoIR oflight irradiation amount in formation of the second alignment film 18 is10%, the light transmittance TR_(LCD) is improved in comparison with theliquid crystal panel in the first embodiment (PT5) having the samestructure as that of this liquid crystal display panel, and the AC afterimage intensity IoS_(AC) is equal to or less than 1.0%. Further, thedifferences ΔTR between the light transmittance TR_(LCD) in the liquidcrystal display panel (PT1) as the first comparative example and thelight transmittance TR_(LCD) in these liquid crystal display panels (PT6to PT9) are +0.21%, +0.27% and +0.32% and +0.35%. The differences aregreater than those in the liquid crystal display panel in the firstembodiment. That is, in the liquid crystal display panel in the secondembodiment, it is possible to further improve the light transmittance incomparison with the liquid crystal display panel in the firstembodiment.

Accordingly, in the liquid crystal display panel in the secondembodiment, it is possible to achieve both prevention of reduction oflight transmittance due to coloration of the first alignment film 8 andsecond alignment film 18 and the suppression of occurrence of afterimage can be achieved, and the advantage of prevention of reduction oflight transmittance due to coloration is higher than that in the firstembodiment.

As described above, the present invention has been specificallydescribed based on the above embodiments. However, the present inventionis not limited to the above embodiments, and various modifications canbe made within a scope without departing from the subject matter.

For example, in the first and second embodiments, as an example of pixelstructure, the pixel electrode 16 and the common electrode 14 arelaminated via the third insulating layer 15 and the pixel electrode 16is closer to the liquid crystal layer 3. However, when the pixelelectrode 16 and the common electrode 14 are laminated, the arrangementis not limited to this example, but the common electrode 14 may becloser to the liquid crystal layer 3. In this case, the common electrode14 closer to the liquid crystal layer 3 has a comb teeth shape.

Further, when the pixel electrode 16 and the common electrode 14 arelaminated via the third insulating layer 15 and an electrode closer tothe liquid crystal layer 3 is comb teeth-shaped, the extending directionof the teeth of the electrode and the number of teeth can be arbitrarilychanged.

Further, in the first and second embodiments, as an example of the IPSmode pixel structure, the pixel electrode 16 and the common electrode 14are laminated via the third insulating layer 15. However, the IPS modepixel is not limited to this example. For example, the pixel electrode16 and the common electrode 14 may be provided on the same surface ofthe insulating layer.

Further, in the first and second embodiments, a so-called transmissiontype liquid crystal display panel is given. However, the presentinvention is not limited to this type of liquid crystal display panel,and is also applicable to reflective type and semi transmission typeliquid crystal display panels.

The invention claimed is:
 1. A liquid crystal display device comprising,a first substrate having a first alignment film, a second substratehaving a second alignment film, a liquid crystal layer disposed betweenthe first alignment film and the second alignment film, a firstelectrode and a second electrode disposed between the first substrateand the first alignment film, and an insulation layer disposed betweenthe first electrode and the second electrode, wherein the liquid crystallayer is configured to be controlled by an electric field which isgenerated between the first electrode and the second electrode, whereinthe first electrode and the second electrode are transparent, whereinthe first alignment film is a photo alignment film, and wherein athickness of the second alignment film is less than a thickness of thefirst alignment film measured at a light transmitting area of the liquidcrystal display device.
 2. The liquid crystal display device accordingto claim 1, wherein the second electrode is a planar shape.
 3. Theliquid crystal display device according to claim 1, wherein thethickness of the second alignment film is greater than or equal to 10 nmand no greater than 50 nm.
 4. The liquid crystal display deviceaccording to claim 3, wherein the thickness of the first alignment filmis greater than or equal to 80 nm and no greater than 130 nm.
 5. Theliquid crystal display device according to claim 1, wherein the secondalignment film is a photo alignment film formed by irradiating aninsulating film with linear polarized light.
 6. The liquid crystaldisplay device according to claim 5, wherein the thickness of the firstalignment film is greater than or equal to 80 nm and no greater than 130nm.
 7. The liquid crystal display device according to claim 1, whereinthe first alignment film is disposed on the second electrode.
 8. Theliquid crystal display device according to claim 1 further compromising:a video image signal line supplying an image signal to the firstelectrode; and a light shielding film overlapping with the video imagesignal line; wherein the light transmitting area doesn't overlap withthe light shielding film.
 9. The liquid crystal display device accordingto claim 8, wherein the light shielding material is a black matrix.